1. Field of the Invention
The present invention relates to a copier, facsimile apparatus, plotter or similar color image forming apparatus and more particularly to an image transferring unit for an image forming apparatus.
2. Description of the Background Art
A color image forming apparatus of the type including four image forming units arranged in a tandem configuration is conventional. In this type of apparatus, bias applying members each face a particular photoconductive drum or similar image carrier with the intermediary of an image transfer belt. Biases are applied to the bias applying members for sequentially transferring toner images formed on the drums to a sheet one above the other.
As for a black-and-white image forming apparatus, various technologies relating to the position of a bias applying means relative to a photoconductive drum have heretofore been proposed. For example, when a drum and a bias applying member are located to face each other with the intermediary of an image transfer belt, part of the image transfer belt upstream of an image transfer nip may be raised to wrap around the drum. This configuration allows a sheet to contact the drum before subject to an electric field for image transfer for thereby reducing toner scattering and defective images ascribable to discharge.
Japanese Patent No. 3,131,126, for example, proposes to wrap part of an image transfer belt downstream of an image transfer nip around a photoconductive drum for the purpose of reducing the amount of discharge in the event of separation discharge and reducing, e.g., reverse image transfer. Japanese Patent Laid-Open Publication No. 6-202497 contemplates to reduce defective images without raising part of an image transfer belt upstream or downstream of an image transfer nip. More specifically, this document teaches a configuration that reduces the nip width between a bias applying member and an image transfer belt for thereby positioning the regions around the inlet and outlet of an image transfer nip and where an electric field acts remote from a bias applying member.
On the other hand, in a tandem, four-color image forming apparatus, accurate register of images of four different colors is a target difficult to tackle. In this respect, when part of the image transfer belt upstream of the image transfer belt is raised, as taught in Japanese Patent mentioned earlier, the leading edge of a sheet is apt to abut again the drum on entering the image transfer nip, resulting in a fine change in speed of the order of several ten micrometers and therefore in color shift. This is particularly true when the sheet is relatively thick.
As for image transfer in a tandem, four-color image forming apparatus, Japanese Patent Laid-Open Publication 6-95536, for example, proposes to shift an image transfer roller from a position where it faces a photoconductive drum via an image transfer belt to the downstream side in the direction of sheet conveyance. According to the above document, this configuration obviates, e.g., a change in the speed of the drum when a sheet enters an image transfer nip due to pressure acting on the image transfer roller.
The scheme taught in Laid-Open Publication No. 6-202497 is successful to reduce an impact to act on the drum ascribable to the leading edge of a sheet. However, it is difficult to maintain the bias applying member and image transfer belt in uniform contact because the contact width thereof is reduced. Consequently, image transfer tends to be defective due to short parallelism between the drum and the bias applying member or the bend of the bias applying member. Further, by simply reducing the contact width of the bias applying member and image transfer belt, it is impossible to wrap part of the belt upstream of the image transfer nip around the drum over a desired width and therefore to sufficiently obviate abnormal discharge at the position upstream of the image transfer nip.
The configuration disclosed in Laid-Open Publication No. 6-95536 has a problem that a current for image transfer cannot flow unless the charge roller is raised by pressure high enough to overcome the tension of the image transfer belt. Such high pressure translates into high nip pressure between the image transfer roller and the drum. As a result, it is likely that an image is locally lost as if vermiculated or that a toner image formed on a sheet at a preceding station is peeled off. Moreover, the leading edge of a sheet exerts a load on entering the image transfer nip due to the high nip pressure, bringing about fine color shift.